1. Technical Field
The present invention is related to inductively coupled RF plasma reactors used in semiconductor processing, of the type employing a coiled antenna to couple RF power to the plasma reactor chamber, and in particular to methods for turning the RF power circuit (including the coil antenna) in response to impedance changes in the plasma.
2. Background Art
An inductively coupled plasma reactor typically has a coiled antenna adjacent the plasma reactor chamber and an RF generator connected through an impedance match circuit and a 50 Ohm cable to the coiled antenna. As disclosed in U.S. patent application Ser. No. 08/277,531 filed Jul. 18, 1994 by Gerald Yin et al. entitled PLASMA REACTOR WITH MULTI-SECTION RF COIL AND ISOLATED CONDUCTING LID and assigned to the assignee of the present application, such an inductively coupled plasma reactor may have a ceiling over which the coiled antenna is wound. In carrying out semiconductor processes such as metal etching, as one example, the amount of power applied to the plasma in the chamber is a critical parameter and is selected with great care. Any significant deviation from the selected power level may so change the process as to reduce product yield, as is well-known to those skilled in the art. For example, the plasma density, which affects etch rate, is a function of the power coupled to the plasma.
The RF impedance presented by the plasma fluctuates during processing. Unless the RF match circuit is able to compensate for such fluctuations, an RF mis-match arises between the RF source and the plasma, so that some of the RF power is reflected back to the source rather than being coupled to the plasma. Plasma impedance fluctuations during RF plasma processing on the order of 5% are typical. In order to enable the RF match circuit to compensate or follow such fluctuations and maintain an RF match condition, the RF match circuit includes variable capacitors controlled by electric motor servos governed by an RF detector circuit. The RF detector circuit responds to changes in reflected power by changing the variable capacitors to maintain RF match between the RF source and the plasma.
One problem with this approach is that the electric motor servos and variable capacitors are expensive and heavy. A related problem is that it is difficult to compensate for large fluctuations in plasma impedance using electric motor servos and variable capacitors. A further problem is that the electric motor servos are relatively slow and unreliable (being subject to mechanical breakdown). What is needed is a device for instantly responding to wide fluctuations in plasma impedance to maintain RF match without employing heavy or expensive mechanical devices or variable capacitors.